Our EMS company gives out helicopter lapel pins as an advertising promo.
Some years ago my Mom was wearing one I had given her as she stood in line at a grocery store. The guy in line behind her noticed the pin and asked her about it. When she told him what I did for a living, he asked her how I became a helicopter pilot.
She told him I was Army trained and a Viet Nam veteran, and he said "I was a grunt. You tell him the sound of that Huey headed our way to pull us out of the field was the most wonderful sound in the world!"
And what a sound!
In some ways, like a heartbeat.
I have 3,000 hours beneath the rotor of various models of the UH-1.
The rotor on the UH-1H model Huey was 48 feet 3 inches in diameter.
Each time that two-bladed rotor swung around, 24 feet of it was takin' a huge bite out of the air.
The red-line on the airspeed indicator was 120 knots/138 miles per hour. The rotor itself was turning at over 400 miles per hour at the tip. When it came around on the advancing side, it was going fast enough under some conditions to begin to build up a pressure wave, associated with the speed of sound.
That's what gave the Huey it's distinctive sound signature.
One of the reasons helicopters are not so noisy today is that rotor diameter has been reduced, lessening the speed at the tip. Most manufacturers are making multi-bladed systems, which decrease the amount of work the individual blade needs to do, therefore lowering sound levels.
When you hear a helicopter flying by, in most cases the loudest noise you hear is the tail rotor. It turns at 6-7 times the R.P.M. of the main rotor, and makes a high-pitched hum.
The main rotor is generally the second loudest thing on the helicopter. In most machines it is a lower pitch buzzing sound.
On top of all this you add the noise of the engine. In the case of a turbine, it is barely noticeable on top of rotor noise. In helicopters powered with reciprocating engines you can identify the engine sound, but it generally is not objectionable..........more like the sound of your neighbor mowing his lawn at a distance.
I was always aware of the noise footprint of the Huey. It was such a noisy machine, when I flew over cities or late at night, I tried to fly at altititudes that would dissipate the noise.
When I hear one now.......no matter where I am or what I'm doing, I'll stop to watch it pass.....
everything comes to a stop until the aircraft is out of sight and out of earshot.
The Old Huey is a sweetheart, and I'll love her 'til my dying day.